There’s nothing to be concerned about, in the psychiatric sense, when it comes to someone using religious beliefs to explain awful behaviour and views.
Photograph: Mary Turner/Reuters

If someone told you, in all seriousness, that they talk to invisible beings who control the universe, you’d probably back away slowly, nodding and smiling, while desperately looking for the nearest exit or escape route. If this person then said they wanted to be in charge of your life, you’d probably do the same, but more urgently, and with a view to finding the nearest police officer.

Well, it shouldn’t be, because as they say, “You talk to God, you’re religious. God talks to you, you’re psychotic.” That’s a line from the TV show House MD, delivered by the eponymous acerbic medic played by Hugh Laurie. But variations of this comment have been made many times over the years. However, while it is seemingly intended to highlight the double-standards inherent in accepting someone’s religious views as fine while dismissing similarly unscientific claims as signs of mental disturbance, there is a valid reason for this apparent inconsistency.

Psychosis is defined as a loss of contact with reality, and can manifest in numerous ways. It’s alarmingly common: our big, bulky, complex brains are unnervingly vulnerable to internal disruption from a very wide range of illnesses or physical ailments, so much so that it’s regularly labelled a “diagnosis of exclusion”; you have to rule out numerous other problems before you can diagnosis psychosis in its own right.

Psychosis typically manifests by people experiencing hallucinations (perceiving something that isn’t actually there) and delusions (unquestionably believing something that is demonstrably not true). Hallucinations can be straightforward; if someone is repeatedly saying there’s a talking bear in the room demanding french fries, it’s relatively easy to determine whether this is the case or not, usually by looking around to check if there is indeed a talking bear in the room with you. It’s the sort of thing you’d notice. If there isn’t one there, the person is very likely to be hallucinating.

Delusions are trickier: it’s not about what someone perceives, but what they believe. Delusions have many forms, like grandiose delusions, where an individual believes they’re far more impressive than is the case (e.g. believing they’re a world-leading business genius despite being a part-time shoe shop employee), or the more common persecutory delusions, where an individual believes they are being relentlessly persecuted (eg everyone they meet is part of some shadowy government plot to kidnap them). These delusions tend to be very resistant to argument, no matter how blatant the evidence to the contrary: “If you’re a world-leading business guru, why do you flip burgers for a living?” “It’s all part of my brilliant plan, you wouldn’t understand”, or “That’s not a secret government spy, it’s an old man walking his dog” “Well you WOULD say that, you’re in on it!” And so on.

That’s actually one of the signs of delusional beliefs: they’re very resistant to being challenged, no matter how inconsistent they are with reality. Because the brain isn’t “working” like it should, logic and reason aren’t as potent they might otherwise be.

But then, that begs the question, why do religious beliefs get a free pass? People are very resistant to those being challenged too. And believing that there’s a kindly-but-all-powerful father figure in the sky who watches and judges everything you do and his son who died but came back to life two millennia ago is going to return any minute, surely that’s no less likely than someone being targeted by a shadowy government conspiracy? It’s substantially less likely, in actual fact. What gives?

“I can believe that a stone age man built a wooden boat large enough to house every species on Earth, but climate change just seems a bit of a stretch to me” Photograph: John Minchillo/AP

Well, delusions are believed to stem from anomalous activity in the brain’s system for interpreting what does happen and what should happen. The brain essentially maintains a mental model of how the world is meant to work, and what things are meant to happen and when. Beliefs, experiences, expectations, assumptions, calculations; all are combined into a constantly-updated general understanding of how things happen, so we know what to expect and how to react without having to figure everything out from scratch each time. Luckily, the brain is usually quite good at filtering out irrelevant information and occurrences that would otherwise challenge this model of how the world works.

Delusions are what happens when, due to illness or other disruption, this delicate system fails, and things we perceive that would usually be dismissed as innocuous or irrelevant end up being processed as far more significant, and our belief system alters to accommodate it, however wrongly.

But the thing is, our brains don’t come with an understanding of the complex science of how the world works already preinstalled, like Windows 10 on a new laptop. This mental model of the world is built up over time, from life experiences and other learning. So, if you’ve been raised in an environment where you’re told by everyone and everything that there’s a kindly deity in the sky, or that the world is 6,000 years old, or that there are thousands of multi-armed gods controlling the world, or whatever, then why wouldn’t you believe it? There’s nothing that you experience on a day-to-day level that contradicts this, so your mental model of the world is fine with it.

That’s why delusions are only diagnosed if they’re not consistent with the person’s existing belief system and views. A devout creationist talks to God while in church, that’s fine. An avowedly atheist lawyer starts doing it in the middle of a meeting, they’re probably delusional. If both of them suddenly started saying the world is going to end in 30 minutes because of angry frogs living in the sun, they’d both be considered delusional.

Unless that’s mentioned in the Bible somewhere? I admit I haven’t read it in a while.

This article was adapted from The Idiot Brain, Dean Burnett’s debut book, available now in the UK and US and elsewhere.